Architectural-element analysis and depositional models for pre-vegetation braidplain and braid-delta environments, with modern analogues

Pre-vegetation landscapes that blanketed the continents before the emergence vascular plants in the late-Silurian are proposed habitats for the earliest terrestrial biota and are analogous to martian setting thought to have potentially hosted life. Analysis of the middle member of the Wood Canyon Formation, a Cambrian age sandstone, reveal new details about terrestrial pre-vegetation environments. In fluvial middle-member stratigraphy, units are defined by stacking patterns of three facies associations (FA1-3). In FA1, stacked cosets, interpreted as braidplain barforms and channel fills, preserve vertical- and downstream-accretion elements under unimodal paleoflow. Floodplains, represented by FA2, include red-orange intervals of fine- to medium-grained sandstone and thinner sets of cross-bedding than FA1. Possible aeolian beds of FA3 preserve broad festooned trough cross-strata that average 23 centimeters in thickness. A nearby stratigraphic section of middle member preserves the development of a fluvial-dominated braid delta at the terminus of the braidplain. In contrast to other pre-Devonian braid-delta strata that are largely absent mudrock, the majority of the braid-delta system contains over 5% mudstone. The 20 meters of braidplain-to-delta transition strata (FA4) are largely similar to those of fluvial middle-member sections. Each braid-delta facies association (FA5-7) preserves high sinuosity paleocurrent indicators, 6–12% mudstone, and symmetrical sand waves. Fluid-mud deposits found primarily in FA6 indicate the presence of a maximum turbidity zone. Trace fossils found only in the most distal braid delta suggest metazoans could not tolerate fresh water or the variable salinity of the braid delta. Global surveying of 786 Holocene braid deltas provides an understanding of present-day, pre-vegetation analogues. Systems develop at all latitudes, with the exception of the Antarctic, but are densest in the periglacial latitude bands. A novel classification of braid-delta morphotypes, separates deltas by flow characteristics, dominance of wave, river, or tide modification, and confinement style. Wet, river-dominated, linear or radial morphotypes and wet or dry, wave-dominated, linear morphotypes represent two-thirds of the dataset. Geomorphic maps for each morphotype, 38 km² in total area, detail a range of possible facies components. Importantly, 63% of the observed braid deltas are not the distributary forms that are common in meandering-river deltas.

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